The first tropospheric wind profiler observations of a severe typhoon over a coastal area in South China

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The first tropospheric wind profiler observations of a severe typhoon over a coastal area in South China Lei Li, 1 Pak Wai Chan, 2 Honglong Yang, 1 Rong Zong, 1 Xia Mao, 1 Yin Jiang 1 and Hongbo

1 The first tropospheric wind profiler observations of a severe typhoon over a coastal area in South China Lei Li, 1 Pak Wai Chan, 2 Honglong Yang, 1 Rong Zong, 1 Xia Mao, 1 Yin Jiang 1 and Hongbo Zhuang 1 1 Shenzhen National Climate Observatory, Meteorological Bureau of Shenzhen Municipality, China 2 Hong Kong Observatory, China For weather monitoring and forecasting purposes, it is useful to have continuous measurements of upper level winds, at least within the troposphere. For that purpose, a boundary-layer type wind profiler, working at a radio wave frequency of about 1.3GHz, was introduced to Hong Kong in the mid-199s. Compared with conventional radiosonde data, which are only available a few times per day, wind profilers provide unprecedented views of vertical wind profiles at a high temporal frequency. In recent years, this type of wind profiler has been introduced to many areas in the Pearl River Delta region, including Guangzhou and Shenzhen, to form a network of profilers to observe the upper level wind profiles. The boundary-layer wind profilers have a nominal measurement height of about 3km above ground, reaching 6km above ground in rain. However, using a special configuration, this type of profiler can measure winds up to 9km above ground in rain (Chan and Yeung, 23). Wind measurements in the middle and upper troposphere are useful in the monitoring of heavy rain and tropical cyclones, including the passage of westerly waves in the middle troposphere for heavy rain episodes and the descent of jets from the upper troposphere in association with typhoons (Yeung, 1998). Wind measurements at higher levels are only available in rain from boundary-layer profilers, and these data are unavailable in less humid situations. The quality of the wind data from boundary-layer profilers has been demonstrated by comparison with radiosonde data (Chan and Yeung, 23). In order to improve wind monitoring in the middle and upper troposphere, a tropospheric wind profiler (TWP) was introduced to Xi Chong, Shenzhen in 2. The location of this profiler is shown in Figure 1, and the appearance of the profiler is shown in Figure 2. Shortly after the testing and commissioning of the TWP in the middle of August 213, severe typhoon Usagi crossed Shenzhen in late September 213. This paper documents the observations from this TWP during the Usagi episode. We include the first continuous middle to upper tropospheric wind profiles available from the TWP for the severe typhoon Usagi; raw spectrum data from the TWP was processed by the National Center for Atmospheric Research (NCAR) Improved Moments Algorithm (, see Morse et al., 22, for technical details) to produce 1min mean winds continuously. The quality of the -processed wind profiles and that of the direct data output from the TWP itself (based on the algorithm from the manufacturer) are studied by comparison with radiosonde data, boundary-layer data from the profiler station at Sham Shui Po, Hong Kong (location given in Figure 1), as well as aircraft data. This is the first comprehensive study of the quality of the wind profile data from a TWP radar based on different types of upper-air measurements. Equipment The TWP at Xi Chong (model: Vaisala LAP- ) operates at a microwave frequency of 51MHz. It measures the three components of the wind by tracking the changes in refractivity associated with atmospheric discontinuities. It operates in a three-beam configuration: one vertical beam and two oblique beams at an angle of 15 from the vertical. The two oblique beams point towards the east and the south. The maximum measurement range is about km, starting at about 1km. The gate height spacing (i.e. the vertical spacing between two consecutive measurement heights) is about 145m. Weather January 215, Vol. 7, No. 1 Latitude ( ) Latitude ( ) Shenzhen Xi Chong: tropospheric wind profiler Sham Shui Po: boundary layer type wind profiler King s Park: radiosonde station Longitude ( ) Longitude ( ) Figure 1. The track of Severe Typhoon Usagi: the typhoon s whole lifespan, the equipment locations. 9

Radiosonde ascents were launched at King s Park (location shown in Figure 1), usually three times per day: at, and UTC. Data were available every 2s.

2 Weather January 215, Vol. 7, No. 1 Wind profiler observations of a severe typhoon Figure 2. The tropospheric wind profiler at Xi Chong, Shenzhen. The wind measurements from three kinds of platform are used to study the quality of the data from the Xi Chong profiler: radiosonde, boundary-layer profiler and aircraft. Radiosonde ascents were launched at King s Park (location shown in Figure 1), usually three times per day: at, and UTC. Data were available every 2s. On tember 213, an additional ascent was made at UTC because of the proximity of the typhoon. There are several boundary-layer type wind profilers in the Pearl River Delta region. In this study, the profiler at Sham Shui Po, a Vaisala LAP-3 operating at a frequency of 99MHz, was used (its location is shown in Figure 1). The profiler s minimum measurement height is 3m and its maximum measurement range is approximately 9km. The gate height spacing is m. It also measures in a three-beam configuration. Narrative of the typhoon event and weather radar observations The track of Severe Typhoon Usagi is shown in Figure 1, including the track over its whole lifespan and the part of its track in the Hong Kong/Shenzhen area. The typhoon entered the 8km warning area of Hong Kong on the morning of 21 September 213 and continued to intensify as it moved west-northwestward across the northern part of the South China Sea. It made landfall at Shanwei on the evening of tember and continued to move in a generally westward direction across Shenzhen. It was closest to the Xi Chong profiler at around midnight Hong Kong time (HKT: 8h ahead of UTC) on tember, with its centre approximately 5km to the north of the profiler. In this study, the data for the period 21 tember are considered. Figure 3 shows weather radar imagery from Hong Kong at a height of 3km above mean sea-level when Usagi was very close to the TWP at Xi Chong. Figure 4 and. The zoomed-in versions are shown in Figure 5. For the direct output from the Vaisala data processing software, raw wind data measurements were obtained as 5min averages, and six of these 5min averages were used to create a 3min average as the final output. The 5min averages are subject to errors due to the short time interval of the average. A longer term average of 3min is considered necessary to obtain more reliable wind data. As a result, in the present study, directly output 3min averages were used instead of the average of two 5min average wind values to get a 1min average that would be subject to error. In fact, the 5min averages, as intermediate values in the calculation of the overall 3min average, were not stored in the computer system of the TWP. Outputs were made at 1min intervals from the algorithm, similar to that for the boundary-layer profiler. Because of the high density of the 1min mean outputs in the time-height plot, only the 1min mean winds at 3min intervals are shown in Figure 4 and. By comparing Figure 4 and, it can be seen that the horizontal wind data from both sources are very similar. Thus it can be seen that wind data from the TWP radar at 1min intervals can be used from a sophisticated data processing algorithm such as. At around 23 HKT on tember, the wind direction changed abruptly during the passage of Usagi, with northerly winds followed by southerly winds, and the axis of the wind direction change did not seem to be very much tilted with height. As such, the intensity of the typhoon remained quite strong after it made landfall and moved past Shenzhen. There are two interesting features in the horizontal wind profiles. Firstly, before the time of the severe typhoon s passage, there were winds of approximately 5kn at all heights. This wind strength was not present in the southerly wind direction, following the passage of the typhoon, probably because it was weakening during its passage over Shenzhen. The second feature also points to this trend of weakening: the gale force winds (coloured red in Figure 4 and ) appeared to descend from a height of km in the southwesterly to southerly wind directions, and eventually disappeared in the early morning of tember 213. This shows that the typhoon dissipated rather rapidly over land. This is also consistent with the wind trends from surface weather stations in the region (not shown). As a comparison, the height-time plot of wind data from the Sham Shui Po wind profiler is also given in Figure 4(c). It can be seen that the measurement range of this profiler is generally lower in comparison 1 Wind observations The time-height plots of horizontal winds as measured from the direct output of the TWP and the algorithm are shown in Figure 3. Radar picture of Hong Kong when Usagi was moving across Shenzhen.

11 1 9 8 7 6 5 4 3 2 1 11 1 9 8 7 6 5 4 3 2 1 Wind

Time-height plot of winds from the wind profiler: direct

(c) The corresponding plot for Sham Shui Po wind profiler.

11 11 1 1 9 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 (c) 11 1 9

3 Wind profiler observations of a severe typhoon (c) Weather January 215, Vol. 7, No Wind Speed [kn] Figure 4. Time-height plot of winds from the wind profiler: direct data output from LAP-, and data processed by. (c) The corresponding plot for Sham Shui Po wind profiler. The colour scale is given at the bottom of each figure (c) Wind Speed [kn] Figure 5. Zoomed-in versions of data plots from Figure 4. 11

4 Weather January 215, Vol. 7, No. 1 Wind profiler observations of a severe typhoon Radiosonde LAP Radiosonde LAP Figure 6. Root-mean-square error of wind speed and wind direction of LAP- and in comparison with radiosonde data Wind profiler LAP Wind profiler LAP Figure 7. Same as Figure 6 but based on the wind measurements from the boundary layer type wind profiler at Sham Shui Po, Hong Kong. with that of the TWP, except at the time at which it was raining (when the tropical cyclone s centre was close to Hong Kong/ Shenzhen). The quality of the wind data The typhoon case offers a good opportunity to check the quality of the horizontal wind data from the TWP because, with the passage and weakening of the typhoon over land, the wind measurements cover a rather wide range of wind directions and wind speeds. As a first check, the TWP data from the two sources (direct output from the LAP- and from ) are compared to the radiosonde data collected at the King s Park location over the period September 213. There were a total of 1 radiosonde ascents in this period for comparison purposes. Root-mean square errors (RMSEs) for wind speed and wind direction were calculated. The results are shown in Figure 6. In general, the errors from the two algorithms are comparable with each other, again showing that it is possible to obtain 1min mean wind data from the TWP with a sophisticated data processing algorithm. For wind speed, it is interesting to note that the error is smallest in the middle to upper troposphere, between 4 and 1km. The error of is slightly larger above 1km. For wind direction, the error is smallest at a similar height range: between 2 and 8km. For direct profiler output, the error value fluctuates considerably at heights above 8km. The errors associated with, on the other hand, are more uniform, although they can be large at times. The RMSE of data from the Sham Shui Po profiler is shown in Figure 7. Only those gate heights with a least 1 valid data points are considered because wind data at greater heights are not always available from the boundary-layer wind profiler. In general, the errors from the two algorithms are very close. The variation with height shows similar trends and the error values are normally smaller within the lower troposphere, between 1 and 4km above sea-level. The wind data from and the tropospheric wind profiler were also compared with aircraft data. The results are shown in Figure 8. The data from 246 departing flights from Hong Kong International Airport (HKIA) are considered. The RMSE of wind speed of is lower than that of the LAP- at altitudes between 4 and 1km. The two algorithms have similar RMSE values for wind direction, though there is a tendency for the error of to be slightly larger. To the knowledge of the author, there has been a limited number of reports on the comparison between data from commercial jets and wind profiler data, particularly in association with a tropical cyclone. The spectrum of the TWP consists of the returned power as a function of the frequency shift (at a particular height) and height. The traditional signal processing of the TWP spectrum, as used in the direct output from the TWP, considers the return signal at each height individually., on the other hand, is a more sophisticated signal processing algorithm that considers the spectrum in two dimensions: not only at a particular height, but also continuity at the various heights. This makes it possible to pick out the frequency shift from the atmospheric return with higher accuracy. Vertical velocity and spectrum width The time-height plot of vertical velocity from the TWP is presented in Figure 9.

14 1 8 6 4 2 Flight data LAP 5 1 15 2 25 14 1 8 6 4 2 Figure 8. Same as Figure 6 but based on the aircraft data.

5 Flight data LAP Figure 8. Same as Figure 6 but based on the aircraft data. Flight data LAP a height of approximately 4km above sea-level, which may be an indication of the rising of the height of tropical cyclone single boundary layer. Moreover, it can be seen from Figure 9 that there are many horizontal lines of higher EDR 1/3 values. The interpretation of such structures requires further investigation. Conclusions This paper documents the first observations from a TWP over southern China for a landfalling tropical cyclone. A number of new findings come from the study. First of all, using a sophisticated algorithm such as, it is possible to obtain high quality 1min mean wind profiles from the TWP, instead of having wind profiles at only 3min intervals. Moreover, the RMSE of the horizontal wind data was established by comparing with radiosonde, boundarylayer wind profiler and aircraft data. To the knowledge of the authors, this is the first time that the wind fields associated with a typhoon, measured using a TWP, have been studied using such a comprehensive dataset. Furthermore, the vertical velocity from the typhoon was studied for the first time, as opposed to having only the fall velocity of raindrops from a boundary layer type wind profiler. The turbulence intensity from the typhoon was studied at the same time, to reveal the boundary-layer structure of the tropical cyclone. In summary, the TWP provides unprecedented views of the tropical cyclone that could not be obtained by the meteorological equipment previously used in the region. It will be used to collect more observations of typhoons, as well as other severe weather phenomena such as rainstorms, in the future. Wind profiler observations of a severe typhoon Weather January 215, Vol. 7, No. 1 Figure 9. Vertical velocity and cube root of eddy dissipation rate from Xi Chong wind profiler as processed by algorithm. It shows the vertical motion of the air associated with the typhoon, as opposed to mainly the fall velocity of raindrops at times of rain from a boundary-layer wind profiler. It can be seen from Figure 9 that, in the period between the evening of tember and noon on tember, there was upward motion in the lower troposphere (up to about 5km) which subsequently changed to downward motion until the passage of the centre of the typhoon. The downward motion reached a magnitude of about 2 4ms 1. With the passage of Usagi, the motion in the lower troposphere changed to an updraught again. This is the first observation of the vertical motion associated with a typhoon over southern China by a TWP radar. Some interesting features can also be seen from the cube root of the eddy dissipation rate (EDR 1/3 ) as calculated from the algorithm (Figure 9). The EDR 1/3 is calculated from the spectrum width data provided by the TWP. The higher the value of EDR 1/3, the more turbulent the flow, with the highest rates normally observed in the boundary layer. From Figure 9, it can be seen that the maximum EDR 1/3 reaches only about.16m 2/3 s 1, which is not particularly high (for comparison purposes, for moderate turbulence, as observed by aircraft, the EDR 1/3 is in the region of.3m 2/3 s 1 ). Around the time of the passage of the typhoon, higher values of EDR 1/3 rose from a height of approximately 2km to reach References Chan PW, Yeung KK. 23. Experimental extension of the measurement range of a boundary layer wind profiler to about 9km. Twelfth Symposium on Meteorological Observations and Instrumentation, Long Beach, CA, 1 13 February 23. Morse CS, Goodrich RK, Cornman LB. 22. The method for improved moment estimation from Doppler spectra. J. Atmos. Oceanic Technol. 19: Yeung KK Use of wind profiler in severe weather monitoring. Meteorol. Z. 7: Correspondence to: Pak Wai Chan pwchan@hko.gov.hk 215 Royal Meteorological Society doi:1./wea

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